1. Field of the Invention
The present invention relates to a compound semiconductor switching circuit device, and relates particularly to a compound semiconductor switching circuit device that protects from electrostatic breakdown a logic circuit provided for a switching circuit device.
2. Related Background Art
In many cases, mobile communication devices, such as mobile phones, employ a microwave frequency in a GHz band, and a switching element for switching these high frequency signals tends to be employed for an antenna switching circuit or a transmission/reception switching circuit. A field-effect transistor (hereinafter referred to as an FET) that employs gallium•arsenide (GaAs) is frequently employed as such a switching element, and because of this trend, and developments of monolithic microwave integrated circuits (MMICs) have been proceeded, in which the switching circuits are integrated.
For a conventional compound semiconductor, in order to protect a GaAs FET from the electrostatic breakdown, a pn junction diode is connected between terminals that are vulnerable to the electrostatic breakdown. This technology is described, for instance, in “Ion implantation type low-noise dual gate GaAs MESFET”, Yasuo Miyawaki and three others, SANYO TECHNICAL REVIEW, Sanyo Electric Co., Ltd., August 1986, Vol. 18, No. 2, pp. 76-84.
When a logic circuit (an inverter) is connected to a switching MMIC, the number of control terminals can be reduced.
However, an enhancement type FET (hereinafter referred to as an E-FET) that constitutes a logic circuit is small and extremely vulnerable to static electricity. In order to absorb noise and prevent oscillation, a capacitor Ci is connected to an input signal side of the logic circuit, and a capacitor Cr is connected to an inverted signal side thereof. These capacitors Ci and Cr are also vulnerable to static electricity.
Therefore, generally, for the conventional compound semiconductor, a protecting diode, such as a pn junction diode, is connected so that the GaAs FET will not be destroyed by external static electricity.
However, during processes for fabricating the compound semiconductor device, a p-type impurity area for the pn junction diode must be formed separately. Since this process complicates the manufacturing processing, an increase in the manufacturing costs becomes a problem. In addition, a space for arranging the pn junction diode is also required, so that a size of a chip is increased and there is a further rise in manufacturing costs.
Furthermore, for a microwave device, parasitic capacity will be increased and the characteristic deteriorated due to a connection of the protecting diode. That is, in a case involving the logic circuit of the switching MMIC, there has been a problem that a switching speed will be reduced.